Power management mechanism for loop powered time of flight and level measurement systems

ABSTRACT

A loop powered level measurement and time of flight ranging system with power management. The level measurement system includes a controller, a transducer for distance ranging, a power supply, and a power management stage. The power management stage comprises a shunt current regulator and a circuit for charging a storage capacitor. The shunt current regulator shunts excess current from the current loop and the circuit charges the storage capacitor. The charging of the capacitor is monitored and stored energy is applied to the transducer to make a level measurement. During charging, the controller continues to operate a user interface and communication module.

RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119 of foreignpatent application Ser. No. 2,406,298 filed on Sep. 30, 2002 in Canada,which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to time of flight ranging systems andlevel measurement systems, and more particularly to a power managementmechanism and technique for loop powered level measurement systems.

BACKGROUND OF THE INVENTION

Loop powered level measurement systems operate on a 4-20 mA currentloop, hence the name loop powered. The circuitry for the levelmeasurement system, i.e. the load, is typically designed to operate atless than 4 mA. The current loop provides a terminal voltage in therange 12-30V, but is nominally 24V.

To take a measurement, power is applied to the transducer and thereflected pulses are detected and the distance to the reflective surfaceis calculated or measured. If more than 4 mA is needed to make ameasurement, then energy taken from the current loop is stored untilthere is enough to make the measurement. In addition, to make rapidmeasurements, more current from the loop is also needed. As the currentin the loop increases, the speed of measurement also increases. Sincethe power available from the current loop is less than the powerrequired to continuously operate the level measurement device, the levelmeasurement device is operated intermittently. In a typical levelmeasurement system, measurements are taken once every second up to onceevery five seconds.

Accordingly, there remains a need for power management in the field ofloop powered level measurement or time of flight ranging systems.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a mechanism and method for powermanagement in a level measurement or time of flight ranging system.

In a first aspect, the present invention provides a level measurementsystem, the level measurement system is powered by a two wire loop, thelevel measurement system comprises: (a) a transducer for emitting energypulses and detecting reflected energy pulses; (b) a controller having acomponent for controlling the transducer, and a component fordetermining a level measurement based on the time of flight of thereflected energy pulse; (c) a power supply having an input port forreceiving power from the loop, and a component for producing an outputvoltage; (d) a power management unit coupled to the loop, the powermanagement unit having an output coupled to a storage capacitor forcharging the storage capacitor, an input port for receiving excess powerfrom the loop; (e) the transducer including an input for receivingenergy from the storage capacitor under the control of the controller.

In another aspect, the present invention provides a level measurementsystem, the level measurement system is powered by a current loop, thelevel measurement system comprises: (a) a transducer for emitting energypulses and detecting reflected energy pulses; (b) a controller with acomponent for controlling the transducer, and a component fordetermining a level measurement based on the time of flight of thereflected energy pulse; (c) a power supply having an input port coupledthe current loop for receiving current at a voltage level, and the powersupply having a component for producing an output voltage for poweringthe controller; (d) a power management unit coupled to the current loop,the power management unit having an output coupled to a storagecapacitor for charging the storage capacitor, an input port forreceiving excess current from the current loop; (e) the transducerincludes an input for receiving energy from the storage capacitor underthe control of the controller.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is next made to the accompanying drawings, which show, by wayof example, embodiments of the present invention and in which:

FIG. 1 shows in diagrammatic form a loop powered level measurementsystem and power management mechanism according to the presentinvention.

FIG. 2 shows in block diagram form the loop powered level measurementsystem according to the present invention;

FIG. 3 shows in schematic form an implementation for the powermanagement circuitry the level measurement system in FIG. 2.;

FIG. 4 shows in schematic form a circuit implementation for the wastepower supply module; and

FIG. 5 shows in schematic form a circuit implementation for the shuntcurrent regulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is first made to FIG. 1 which shows a loop powered levelmeasurement system with power management according to the presentinvention. The loop powered level measurement system, indicatedgenerally by reference 10, interfaces to a power/communication loop 20,for example a 4-20 mA current loop. The loop powered level measurementsystem 10 is coupled to the current loop 20 at terminals A and B. Aremote receiver, for example a plant control computer, indicated byreference 8 is coupled at the other end of the current loop 20. For atypical 4-20 mA current loop configuration, the loop 20 provides acurrent in the range 4 to 20 mA and a loop voltage in the range 18 to 30Volts. The loop voltage is nominally at 24 Volts and represented as avoltage source with reference 22. The resistance of the loop isrepresented as a resistive element 24 and is typically in the range 0 to550 Ohms. While the loop current is normally in the range 4 to 20 mA,the current may range from 3.6 to 21.6 mA to indicate alarm conditions.

Reference is next made to FIG. 2 which shows in more detail the levelmeasurement system with power management 10 according to the invention.The level measurement system 10 comprises a transducer module 101, acontroller 102, a power supply 104, a shunt current regulator 106, aloop current sensor and amplifier circuit 108, a power management module110 and an energy storage capacitor 112. The level measurement system 10may also include a communication module 114.

The power supply 104 comprises a switching power supply and is designedto consume less than the minimum loop current, nominally 4 mA.

The shunt current regulator 106 is operated under firmware control bythe controller 102 to draw additional current to achieve the desiredcurrent in the current loop 20. One terminal of the shunt currentregulator 106 is connected to terminal A of the current loop 20 and theinput to the power supply 104. The other terminal of the shunt currentregulator 106 is coupled to the power management module 110. The shuntcurrent regulator 106 has a control terminal which is coupled to acontrol output port on the controller 102. The shunt current regulator106 is also used by the power management module 110 to charge thestorage capacitor 112 as described in more detail below. The loopcurrent sensor and amplifier circuit 108 senses the current flowing inthe loop 20 and provides feedback for the shunt current regulator 106and the controller 102. The loop current sensor and amplifier circuit108 may be implemented with a current sensing resistor and an amplifieras shown in FIG. 4.

As shown in FIG. 2, the level measurement system 10 also includes a userinterface module 116. The user interface module 116 comprises a display,for example, a LCD module, and a keypad or touch sensitive overlay onthe LCD.

The transducer module 101 is coupled to a control port and input/outputport on the controller 102. The transducer module 101 includes atransducer, a transmitter stage and a receiver stage (not shown). Thetransducer (not shown) may comprise radar-based technology,ultrasonic-based technology, TDR-based technology (time domainreflective), or other distance ranging technology. Under the control ofa program stored in memory (e.g. firmware), the controller 102 generatesa transmit pulse control signal for the transmit stage in the transducermodule 101, and the transducer (not shown) emits a transmit burst ofenergy, for example, radar pulses directed at the surface of a material50 (FIG. 1) contained in a storage vessel 60 (FIG. 1). The reflected orecho pulses, i.e. the propagated transmit pulses reflected by thesurface 52 of the material 50 (FIG. 1), are coupled by the transducer,for example, a radar antenna or other distance ranging technology (notshown), in the transducer module 101 and converted into electricalsignals by the receiver stage (not shown). The electrical signals areinputted by the controller 102 and sampled and digitized by an A/Dconverter (not shown) and a receive echo waveform or profile isgenerated. The controller 101 executes an algorithm which identifies andverifies the echo pulse and calculates the range, i.e. the distance tothe reflective surface, from the time it takes for the reflected energypulse to travel from the reflective surface 52 (FIG. 1) to thetransducer (not shown) in the transducer module 101. From thiscalculation, the distance to the surface of the material 50 and therebythe level of the material 50 in the vessel 60 is determined. Thecontroller 101 may comprise a microprocessor or a microcontroller withon-chip resources, such as an A/D converter, ROM (EPROM), RAM. Themicroprocessor or microcontroller is suitably programmed to performthese operations as will be within the understanding of those skilled inthe art.

Referring to FIG. 2, all power for the operation of the levelmeasurement system 10 is derived from the current loop 20. The powersupply module 104 comprises a switching power supply which takes itspower input from the current loop 20 and generates the appropriatevoltage levels, e.g. supply rails, for the circuitry, i.e. thecontroller 102, the display and user interface module 106 and the otherelectronic modules in the level measurement system 10. The transducermodule 101 is powered from the waste power supply 110 as described inmore detail below.

The controller module 102 also controls the transmission of data andcontrol signals through the interface with the current loop 20. Thecontroller 102 uses the shunt current regulator 106 to adjust ormodulate the loop current in the range 4 to 20 mA to transmit thecalculated level of the material 50 to the remote receiver or plantcomputer 8 (FIG. 1) connected to the other end of the current loop 20(FIG. 1). As shown in FIG. 2, the level measurement system 10 mayinclude the communication module 114. The communication module 114includes a digital communication modem, for example a HART modem, whichprovides another communication channel between the controller 102 andthe remote computer 8 (FIG. 1) over the wires of the current loop 20.

In operation, the user interface module 116 comprising the displaymodule and the keypad, and the digital communication module 114 are runcontinuously. The display, user interface and communication operationsmay be thought of as primary functions which run continuously. Thetransducer module 101 is operated intermittently to transmit energypulses and detect reflected energy pulses from the surface of thematerial 50 contained in the vessel 60.

The power available from the current loop 20 for the level measurementsystem 10 is given by:(loop voltage−(loop current×loop resistance))×loop currentIn order to achieve the fastest operation rate, all available power fromthe current loop 20 is utilized by the level measurement system 10.

The power management module 110 functions to tap excess power, e.g. loopcurrent>main power supply 104, from the current loop 20, as will bedescribed in more detail below. As shown in FIG. 2, the power managementmodule 110 is coupled to the storage capacitor 112 and the shunt currentregulator 102.

The power management module 110 uses the shunt current regulator 102 tocharge the storage capacitor 112. The power supply 104 is designed tooperate at less than the lower current loop limit, for example, 3 mA,which means that at least 1 mA of loop current is shunted by the shuntcurrent regulator 106 and to the power management module 110 to chargethe storage capacitor 112. As shown, the controller 102 has an inputport coupled to the storage capacitor 112. The controller 102 senses thevoltage on the storage capacitor 112. When the voltage level on thecapacitor 112 is sufficient to power the transducer module 101, thecontroller 102 activates the transducer 101 to perform a levelmeasurement for the vessel 60 (FIG. 1). The controller 102 thendeactivates the transducer module 101 and the capacitor 112 is allowedto recharge using the power management module 110. The level measurementcalculated by the controller 102 is transmitted to the remote computer 8(FIG. 1) as a communication task for the primary functions. It is notnecessary to turn off the controller 102 between measurements, and thecontroller 102 continues to execute the refresh operation and keypadscan functions for the user interface module 116, and the communicationfunction.

Reference is next made to FIG. 3, which shows in schematic form acircuit implementation for the waste power supply module 110. As shownin FIG. 3, the waste power supply 110 comprises a regulator circuit 202and a clamp circuit 204. In the waste power supply 110, terminal 206(PRAW) is coupled to the shunt regulator 106 (FIG. 2) and the storagecapacitor 112 (FIG. 2). Terminals 208 (COM) are connected to the commonrail (COM) for the system 10. One terminal of the loop current sensor108 (FIG. 2, FIG. 4). Terminal 210 (CLAMP*) is coupled to the controller102 (FIG. 2), and driven by the controller 102 during start-up. Terminal212 (PHV) is coupled to terminal A (FIG. 1).

In operation, the regulator circuit 202 tries to maintain PRAW, i.e.terminal 206, approximately 2 Volts below PHV, i.e. terminal 212. As thevoltage on PRAW drops to 2 Volts less than the voltage on PHV, diodesD10 and D11 begin to turn off, which causes transistors Q9 and Q6 toturn. With the transistors Q6 and Q9 on, the impedance between PRAW andCOM_CAP is reduced. This tends to increase the voltage differencebetween PRAW and PHV, thereby maintaining an approximate 2 Voltdifference between PRAW and PHV.

In the operation of the clamp circuit 204, when the terminal 210, i.e.CLAMP*, is LOW, resistors R31 to R36 connected in series with diode D13are coupled across terminal A and the common rail COM. This provides aload while the controller 102 is starting up or initializing. Theterminal 210 or CLAMP* is maintained low during start up, and whenCLAMP* is high the load from resistors R31-R36 is disconnected. Thisload on start up is provided to ensure a current draw greater than 22.6mA from the current loop 20.

Reference is next made to FIG. 4 which shows a circuit implementationfor the loop current sensor and amplifier circuit 108. The loop currentsensor and amplifier circuit 108 comprises a resistor 220 and anoperational amplifier circuit 222. The loop current sensor 108 hasterminal 224 (NEG), terminal 226 (COM), and output terminal 228(LOOP_SENSE). The NEG terminal 224 is coupled to terminal B (FIG. 2) andthe current loop 20 (FIG. 1). The COM terminal 226 is connected to thecommon or return supply rail. The LOOP_SENSE terminal 228 provides theoutput for the loop current sensor 108 and is formed from the output ofthe operational amplifier in the circuit 222. The LOOP_SENSE terminal iscoupled to an input on the shunt current regulator 106 as shown in FIG.5.

Reference is next made to FIG. 5, which shows a circuit implementationfor the shunt current regulator 106. The shunt current regulator 106 asshown in FIG. 5 comprises a Pulse Width Modulator (PWM) circuit 230, anerror amplifier circuit 232, a controlled current source 234, and afilter and voltage drop circuit 236.

The PWM filter circuit 230 has an input terminal (PWM) 240, and anoutput terminal (ANALOG_OUT) 242. The PWM terminal 240 is coupled to anoutput port on the controller 102 (FIG. 2) which is used to set thecurrent level, i.e. between 4 to 20 mA. The ANALOG_OUT terminal 242 iscoupled to the “Modulate” signal port on the HART modem 114 (FIG. 2). Inoperation, the PWM filter circuit 230 low pass filters the PWM signal onthe input terminal 240 to produce a DC voltage, i.e. PWM output, whichis proportional to the duty ratio or duty cycle of the PWM signalreceived from the controller 102 (FIG. 2).

The error amplifier circuit 232 has an input terminal (CURRENT_SENSE)244, terminal (PHV) 246, terminal (PRAW) 248, and terminal (PHFV) 250.The CURRENT_SENSE input terminal 244 is coupled to the LOOP_SENSE outputterminal 228 (FIG. 4) from the loop current sensor 108. The PHV terminal246 is coupled to the terminal A (FIG. 2). The error amplifier circuit232 amplifies the difference between the actual current, i.e. asindicated by the input CURRENT_SENSE 244, and the requested current,i.e. the PWM output from the PWM filter circuit 230 which is generatedfrom the input PWM 240. This difference is used in the feedback loop tocontrol the loop current.

The controlled current source circuit 234 is coupled to the output ofthe error amplifier 232. The controlled current source 234 is connectedto the PHV terminal 246. The controlled current source 234 also includesterminal (PRAW) 248. The PRAW terminal 248 connects to the correspondingPRAW terminal 206 in the waste power supply 110 (FIG. 3). As shown thecontrolled current source circuit 234 includes a number of JFETtransistors 254, indicated individually as 254 a, 254 b, 254 c, 254 d,254 e which are coupled in parallel. The parallel arrangement of theJFET transistors 254 allows for higher saturation currents.

The filter and voltage drop circuit 236 is coupled to terminal PHV 246and includes a BIAS terminal 250 and a terminal (PHVF) 252. As alsoshown, the filter and voltage drop circuit 236 includes an inductor(L11) 256, a diode circuit 258 comprising diodes 260 a, 260 b, 260 c,260 d, and a capacitor (C65) 262. The arrangement of the inductor 256and the capacitor 262 filters noise arising from the power supply 104from feeding back in the current loop. The diode circuit 258 is providedfor clamping the inductor 256 for intrinsically safe, i.e. I.S.,applications as required. The remainder of the circuit 236 as shown inFIG. 5 provides a voltage drop of approximately 0.7 Volts between thePHV terminal 246 and the PHVF terminal 252 as required for operation ofthe HART modem 114 (FIG. 2).

In operation, the less current the main power supply 104 draws from theloop 20, the more loop current flows to the shunt current regulator 106and to the power management module 110 for faster charging of thestorage capacitor 112. The faster the charging of the capacitor 112, theshorter the level measurement cycle. To improve charging efficiency, thepower supply 104 is implemented as a switching power supply in the levelmeasurement system 10. This means that as the loop voltage increases,the loop current drawn by the power supply 104 decreases resulting in anincrease in the loop current to the shunt current regulator 106 and thepower management module 110 for faster charging of the storage capacitor112.

To further improve the charging efficiency of the storage capacitor 112,the microprocessor or microcontroller for the controller 102 may beoperated at a slower clock speed, e.g. under control of the firmware andthe instruction set specific to the microprocessor device. Themicroprocessor requires less current when operating at a slower clockspeed. The primary functions, such as display, user interface andcommunications, can be performed at the slower clock speeds.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Certainadaptations and modifications of the invention will be obvious to thoseskilled in the art. Therefore, the presently discussed embodiments areconsidered to be illustrative and not restrictive, the scope of theinvention being indicated by the appended claims rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

1. A level measurement system, said level measurement system being powered by a two wire loop, said level measurement system comprising: (a) a transducer for emitting energy pulses and detecting reflected energy pulses; (b) a controller having a component for controlling said transducer, and a component for determining a level measurement based on the time of flight of said reflected energy pulse; (c) a power supply having an input port for receiving power from the loop, and a component for producing an output voltage; (d) a power management unit coupled to the loop, said power management unit having an output coupled to a storage capacitor for charging said storage capacitor, an input port for receiving excess power from the loop, a control terminal responsive to a control output from said controller for controlling the charging of said storage capacitor; (e) said transducer including an input for receiving energy from said storage capacitor under the control of said controller.
 2. The level measurement system as claimed in claim
 1. wherein said power management unit includes a control terminal responsive to a control output from said controller for controlling the charging of said storage capacitor.
 3. The level measurement system as claimed in claim 1 or 2, further including a user interface module and a communication module for transmitting level measurement data over the two wire loop.
 4. The level measurement system as claimed in claim 3, wherein said controller operates said transducer intermittently and based on the charging of said storage capacitor.
 5. The level measurement system as claimed in claim 4, wherein said controller operates said user interface module and said communication module continuously.
 6. The level measurement system as claimed in claim 5, wherein said power supply comprises a switching power supply, and wherein the two wire loop comprises a current loop operable between a minimum current level and a maximum current level, and said switching power supply operating at current level less than said minimum current level, and the difference in the current levels being directed to said power management unit for charging said storage capacitor.
 7. A level measurement system, said level measurement system being powered by a current loop, said level measurement system comprising: (a) a transducer for emitting energy pulses and detecting reflected energy pulses; (b) a controller having a component for controlling said transducer, and a component for determining a level measurement based on the time of flight of said reflected energy pulse; (c) a power supply having an input port coupled to the current loop for receiving current at a voltage level, and said power supply having a component for producing an output voltage for powering said controller; (d) a power management unit coupled to the current loop, said power management unit having an output coupled to a storage capacitor for charging said storage capacitor, an input port for receiving excess current from the current loop; (e) said transducer including an input for receiving energy from said storage capacitor under the control of said controller.
 8. The level measurement system as claimed in claim 7, wherein said power management unit includes a control terminal responsive to a control output from said controller for controlling the charging of said storage capacitor.
 9. The level measurement system as claimed in claim 7 or 8, wherein said power supply comprises a switching power supply and said switching power supply operates at a current level below the operating current level for the current loop, and said power management unit utilizes the difference in said current levels for charging said storage capacitor under the control of said controller.
 10. The level measurement system as claimed in claim 9, further including a user interface module and a communication module for transmitting level measurement data over the current loop.
 11. The level measurement system as claimed in claim 10, wherein said controller operates said transducer intermittently and based on the charging of said storage capacitor.
 12. The level measurement system as claimed in claim 11, wherein said controller operates said user interface module and said communication module continuously.
 13. The level measurement system as claimed in claim 12, wherein said power management unit comprises a shunt current regulator coupled to the current loop, and having control terminal responsive to a control output from said controller for shunting current from the current loop.
 14. The level measurement system as claimed in claim 13, wherein said power management unit comprises a current sensor coupled to the current loop and having an output for indicating the current level in the current loop. 